Ch 24 - Directional And Lateral Stability Flashcards

1
Q

Directional Stability

A

Concerned with yaw about the normal axis

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2
Q

Lateral Stability

A

Concerned with roll about the longitudinal axis

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3
Q

Yaw and Roll Couple

A

Yaw can induce roll and roll can induce yaw

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4
Q

Static Directional Stability (N)

A

Is when the AC displays an initial tendency to return to straight flight after a disturbance in yaw. (Weathercocking - aligning itself to the airflow)

Mustn’t be too strong because the AC must allow the pilot to change flight direction easily

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5
Q

Sideslip Angle

A

The sideslip angle (yaw) angle is between the longitudinal axis and the RAF

Airflow coming from the right is a positive sideslip angle

Positive Directional Stability = yawning moment right

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6
Q

Yaw Moment Coefficient (Cn)

A

Fin has the greatest influence.

Cn = N (yawning moment) / q(dynamic p) x S(surface area) x b(wingspan)

Positive Directional Stability = AC will yaw towards the RAF

Neutral = no Cn whether sideslip angle increases/decreases

Negative = yaw away from RAF

EASA - at small and moderate sideslip angles, the AC must have static directional stability, at larger sideslip angles, will usually turn neautral or even negative

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7
Q

Factors affecting Directional Stability: The Fin

A

The biggest effect on Directional Stability. The larger the distance between the CG and the fin, along with how big it is, will produce a yawning moment (restorative) that yaws the AC towards the RAF

Restoring moment larger when:
CG further forwards
Fin surface larger
High IAS
Larger Sideslip angle
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8
Q

Fin Aspect Ratio and Sweepback

A

Straight fin with higher aspect ratio generates more side force for given AoA but has a lower stalling angle.

A swept fin or low aspect ratio fin produces less force for a given AoA but has a great stalling AoA

To lower aspect ratio, add a dorsal fin - increases stalling angle and improves Directional Stability.

Usually swept to the same angle as the wing for high speed flight reasons.

Ventral fins increase Directional stability at higher angles of attack (shielded)

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9
Q

Strakes

A

Long fins with relatively little vertical extent which are attached to lower sides of fuselage to improve directional stability at slow speed

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10
Q

Factors affecting Directional Stability: The Wing

A

Swept wing has a weak stabilising effect.

The into wind wing has a greater effective span which produces more lift which means more induced drag which yaws the AC to the RAF

Directional stability increases with Sweepback Angle

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11
Q

Factors affecting Directional Stability: The Fuselage

A

Has a destabilising effect overall.

Forward of the CG creates a larger destabilising effect

Aft of the CG creates a stabilising (restorative) moment

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12
Q

Static Lateral Stability

A

Positive static lateral stability - initial tendency to return to wings level in a roll

An aircraft must be signed to have positive static lateral stability

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13
Q

Bank Angle

A

Angle between the lateral axis and the Horizontal

The greater the bank angle in straight and level flight the larger the sideslip angle

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14
Q

The Roll Moment Coefficient (Cl’)

A

Cl’ = L(rolling moment)/ q(dynamic p) x S(surface area) x b(wingspan)

Positive roll moments describe a roll to the right when viewed from behind

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15
Q

Sideslip angle and Roll moment relationship

A

Positive Static Lateral Stability will roll in the opposite direction from the sideslip. Sideslip angle +, Rolling moment will be - (Stable)

Lateral stability is shown by a line with a negative gradient

Negative lateral static stability - roll the same direction as the side slip (+,+/-,-)

Neutral static lateral stability will generate no Cl’ whether sideslip angle increases or decreases,

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16
Q

Factors Affecting Static Lateral Stability: Dihedral Effect

A

The lower wing has greater AoA and thus produces more lift which rolls the aircraft back towards equilibrium.

Positive Static Lateral Stability (Acheived by; dihedral, sweepback, high mounted wing, high keel surface)

Anhedral produces the opposite effect which reduces lateral stability

17
Q

Factors Affecting Static Lateral Stability: Sweepback

A

The lower wing has greater effect span than the upper wing so it generates more lift than the upper wing which tends to roll the AC back towards the equilibrium.

18
Q

Factors Affecting Static Lateral Stability: High Wing

A

Aerodynamic centre on the high wing is vertically displaced from the CG so when ever the aircraft is banked, a restoring couple forms between the CG and the AC

Shielded upper wing means that the lower wing has increased upwash and thus the angle of attack of the low wing is increased which rolls the wing back towards equilibrium (restoring)

19
Q

Factors Affecting Static Lateral Stability: High Keel Surface

A

The amount of fuselage side above the CG which is presented to the airflow.

The sideways component of RAF impacts the side of the fuselage and forms a couple with the CG which creates a restorative rolling moment.

Dihedral effect becomes stronger with increases sideslip

20
Q

Spiral Instability

A

When directional stability is strong and the lateral stability is weak.

Wing drops, aircraft sideslips towards RAF, higher wing travels faster and therefore produces more lift, if lateral stability weak, AC then rolls towards lower wing which causes more sideslip, more yaw - all leads into a spiral dive with roll re-in forcing yaw and vice versa

21
Q

Dutch Roll

A

Occurs when the lateral stability is strong and the directional stability is weak.

Initial disturbance sets up a gently oscillation patter of roll, yaw and sideslip.

If an AC suffers from weak directional stability and strong lateral stability, a yaw damper must be fitted. You can not take off if it is broken and must land ASAP if it breaks in flight

22
Q

Effects of Pressure Altitude on Dynamic Stability

A

Dynamic stability reduces with an increase in pressure Altitude because the increases TAS for the same IAS results in a smaller sideslip angle.

At lower altitudes, the aerodynamic damping is greater that means the damping effects oppose control inputs more.

Roll/Yaw or corrective inputs are also less effective lower down